Charge-pumps are circuits that can pump charge upward to produce voltages higher than the regular supply voltage, using capacitors to store energy between stages. Charge is transferred from one stage to the next, usually by means of a chain of diodes. In low-voltage applications, the ‘diodes’ in the chain are generally MOSFETs (metal-oxide semiconductor field-effect transistors) connected so that charge can only flow in one direction, i.e. so that the transistors act as diodes having a certain forward bias voltage. Charge-pumps using semiconductor technology are used extensively in memories and in many power-management integrated circuits. Another important field of use of such charge-pump circuits is in passive RFID (radio-frequency identifier) tags where the AC voltage received by the tag's transceiver is first converted to a DC voltage and then ‘pumped’ or boosted to a level required by circuitry in the tag.
A type of semiconductor charge-pump circuit commonly used is the Dickson's charge-pump, which uses a chain of diode transistor stages along which the charge is pumped, and a capacitor at the output of each stage to store the charge. Here, the term ‘transistor stage’ refers to a distinct circuit configuration, for example the diode transistor and capacitor, which is repeated in the chain. Other circuit elements such as an additional transistor and optionally a capacitor to reduce voltage ripple may be connected between the final transistor stage and the output voltage node. A supply voltage is applied to the first stage, and control signals or ‘pumping clocks’ are applied to ‘pump’ charge onto the capacitors. The output at the final transistor or transistor stage is the ‘boosted’ voltage.
The output voltage that can ultimately be achieved by the charge pump—the voltage gain—depends on a number of factors besides the input voltage, for example the number of transistor stages used, the voltage gain of each transistor stage, the output load, and the current consumed. In conventional circuits, the voltage gain over each stage, or single-stage gain, is limited to a certain extent by the threshold voltage of the transistor. In a typical MOSFET, the threshold voltage can be about 0.3-0.4V. The efficiency of such state-of-the-art circuits is usually only about 18%-20%. As long as the supply voltage and pumping clock voltage levels are considerably greater than the threshold voltage, a satisfactory output voltage can be obtained. However, for low-voltage applications where the supply voltage is not much greater than the threshold voltage, the Dickson charge-pump becomes quite unsuitable since its single-stage gain is then negligible.
In an alternative approach, described in the paper “MOS Charge Pumps for Low-Voltage Operation” (Wu, Chang; IEEE Journal of Solid-State Circuits, Vol. 33, No. 4, April 1998), a charge-pump circuit using charge-transfer switches (CTS) in addition to diode transistors is used to give a higher single-stage gain, since the charge transfer in this circuit is independent of the threshold voltage. However, this type of circuit also has its limitations, namely reverse charge leakage over the charge-transfer switches, which results in a lower boosted output voltage. To overcome this, an additional pair of transistors must be included for each boosting stage in order to completely turn off the charge-transfer switch transistors, resulting in a correspondingly complex circuitry and increased power dissipation.